Li and Armor [Appl. Catal. B3, 55–60, 1993] describe the Co-ZSM-5-catalysed simultaneous decomposition of NO and N2O in the presence of methane or propane and oxygen:2NO+CH4+O2→N2+2H2O+CO22N2O→2N2+O2
It has been found that although NO is still converted with a high degree of conversion at low temperature, the degree of conversion of N2O decreases sharply at lower temperature. A temperature in the range of 450° C. to 500° C. is found to be necessary for a high degree of conversion of N2O (see Li and Armor: Table 1). Furthermore, it is found that the degree of conversion of NO is higher when pure oxygen is used instead of N2O as oxygen source (see Li and Armor: Table 2). On the other hand, the reduction of N2O proceeds less favourably when both methane and oxygen are present (see Li and Armor: Table 3). The highest degrees of conversion are achieved at 500° C.; at this temperature 97% of the N2O and only 30% of the NO are converted. This method thus has the disadvantage that the temperature range in which acceptable conversion of both NO and N2O takes place is relatively narrow. In addition, this temperature is relatively high (450–500° C.). Moreover, the supply of oxygen has an advantageous effect (higher degree of conversion of NO), but also an adverse effect (lower degree of conversion of N2O).
Kögel et al. [Catal. Lett. 51, 23–25, 1998] describe the Fe-MFI-catalysed simultaneous decomposition of NO and N2O in the presence of propane and oxygen. It has been found that although NO is still converted with a high degree of conversion at low temperature, the degree of conversion of N2O decreases sharply at lower temperature (see Kögel et al.: FIG. 2): the maximum degree of conversion of NO (40%) is reached at 300° C. and at this temperature the degree of conversion of N2O is only approximately 5%. Moreover, the undesired CO is formed as a by-product. However, this disadvantage can be eliminated if a Pt-promoted Fe-MFI is used (oxidation of CO to CO2). However, this has an adverse effect on the degrees of conversion of NO and N2O (see Kögel et al.: FIG. 3). A possible combination of this type of catalyst with the catalyst described by Li and Armor is not obvious because, inter alia, the optimum temperatures are very different (300° C. and 450–500° C., respectively).
Perez-Ramirez et al. [Appl. Catal. B, 25, 191–203, 2000] describe a dual-bed catalytic system intended for the removal of nitrogen oxides (NO and NO2) from a gas stream. In the first step NOx conversion takes place with the aid of a Pt/AC catalyst in the presence of propene as reducing agent. A major disadvantage of such a catalyst is that during the conversion of NO the formation of laughing gas takes place as the main reaction. Perez-Ramirez et al. therefore also studied whether this laughing gas formed can be effectively converted into nitrogen and oxygen in a second step in the presence of propene using Fe-ZSM-5. However, the use of these two catalysts has the disadvantage that two reactors have to be used because the optimum temperature ranges for the two catalysts are far apart (Pt/AC: optimum temperature is approximately 200° C.; Fe-ZSM-5: >430° C.; see also Perez-Ramirez et al.: FIG. 10). It is true that the required temperature for the Fe-ZSM-5 catalyst can be lowered to some extent by supplying additional propene to the second reactor (the product stream from the first reactor no longer contains any propene: see page 197, left-hand column, second paragraph; when propene is supplied to the reactor containing Fe-ZSM-5 a higher degree of conversion of N2O is found; see page 199, right-hand column, second paragraph), but not to a sufficiently low value.
The reduction of N2O with the aid of a saturated hydrocarbon and an Fe-zeolite catalyst is described in WO 99/49954. The preferred temperature at which this catalyst converts N2O is below 350° C. It is neither described nor suggested that this catalyst can be used together with another catalyst for the simultaneous removal of nitrogen oxides (NO and NO2) and N2O.
The conversion of NOx with the use of, inter alia, a Co-zeolite catalyst is described in U.S. Pat. No. 5,149,512. Here NOx is understood to be a mixture of at least two nitrogen oxides, including laughing gas (see, for example, column 3, lines 55–59 and column 4, lines 44–48). However, the fact that in this patent NOx is not used to refer to N2O is confirmed by the later publication by Li and Armor in Appl. Catal. B 3, 1993, on page 56. The experiments show only conversions of NO and not conversions of N2O. U.S. Pat. No. 5,149,512 thus does not explicitly describe whether such a catalyst is able to reduce laughing gas. The best degrees of conversion are obtained at 450° C. (see U.S. Pat. No. 5,149,512: Table 2). A combination of this cobalt catalyst for the conversion of NO with, for example, the abovementioned iron catalyst for the conversion of N2O would, on the basis of the data described, give a catalyst combination with completely different operating temperatures for the different conversions (<350° C. and 450° C., respectively) and would probably give degrees of conversion for NO that are too low. Incidentally, on the basis of this patent (U.S. Pat. No. 5,149,512) it would be best to choose a rhodium catalyst for the conversion of NO (see U.S. Pat. No. 5,149,512: Tables 1, 2, 5, 8 and 9, from which it can be seen that in the presence of CH4 and O2 the rhodium catalyst yields a degree of conversion of 55%, compared with 26, 34, 27 and 34%, respectively).
A combination of an iridium catalyst and a platinum catalyst is described in U.S. Pat. No. 5,997,830.
In U.S. Pat. No. 5,524,432 a catalytic reduction of nitrogen oxide (this term includes laughing gas; see U.S. Pat. No. 5,524,432: column 5, lines 12–15) is described where a reducing agent that has to contain methane is used (see U.S. Pat. No. 5,524,432: Claim 1 and column 4, lines 47–56). The advantage of this method is said to be that ammonia is not required (see Example 11, lines 19–21). The catalytic reduction is followed by a catalytic oxidation of residual methane into carbon dioxide and water. The catalyst that is used for the catalytic reduction comprises a crystalline zeolite with an Si/Al ratio of 2.5 or more and the zeolite contains cobalt, nickel, iron, chromium and/or manganese cations (see column 4, lines 37–46). Co-ZSM-5 is explicitly described in Example 5. However, U.S. Pat. No. 5,524,432 does not teach the person skilled in the art that this catalyst is suitable for the reduction of laughing gas.